Signaling is normally done in one of two methods: Channel Associated Signaling (CAS) or Common Channel Signaling (CCS).
Channel Associated Signaling (CAS) is also know as Bit-Robbing or Bit-Robbed Signaling. Bit-Robbing is one of the first and still probably the most widely-used methods of signaling. Bit-Robbing gets its name because the method robs bits from each DS-0 (time slot or channel) within the T1. In every 6th and 12th frame of a superframe, the multiplexer will rob the least significant bit in each DS-0 and use that bit for signaling purposes.
An example would be off-hook and on-hook signaling. The multiplexer will make the least significant bit a 1 to indicate an off-hook condition. For on-hook signals, the multiplexer makes that bit a zero. The far-end demultiplexer can look at every 6th and 12 frame to retrieve this signaling information.
Since the equipment may actually change this bit, that bit can never be used to carry data. In those frames, only seven bits can transmit information for a customer, as the eighth bit may be distorted.
For data services that use bit-robbed signaling, all carriers guarantee only 7 bits per byte. This allows an actual data throughput of 56kbps (8,000 frames times 7 bits equals 56k bits). 56kbps does not harm voice quality and that is why Bit-Robbing is extensively used in voice applications.
The type of framing used also plays a role in how often the bits are robbed from the DS-0. In D4 framing, the bits that are robbed for signaling purposes are called A and B bits. The A bit is the bit robbed every 6th frame, the B from the 12th frame.
ESF Framing uses A, B, C and D bits since the superframe contains 24 consecutive frames instead of 12. The A bit is the Bit Robbed from the 6th frame, the B from the 12th, the C from the 18th and the D bit is the Bit Robbed from the 24th frame.
Bit-Robbed signaling is not the wave of the future. Common Channel Signaling (CCS), currently in use today in ISDN, will probably replace Bit-Robbed signaling. As we can see in Figure 5, Instead of a digital multiplexer robbing bits from each DS-0, CCS allows that multiplexer to take all of the signaling information from all of the DS-0s and put that information on a separate DS-0. The T1.5 configuration of this scenario is:
· 23 DS-0s of customer related data, and
· 1 DS-0 that carries all of the signaling information for the other 23 DS-0s.
In the E1 configuration we have 30 DS-0s of customer related data, and 1 DS-0 that carries all of the signaling information for the other 30 DS-0s.
Each of the DS-0s that carries customer information is called B or Bearer channels. Since no Bit-Robbing occurs, each B channel caries 64kpbs. Used for data purposes at 64kbps, the customer can transmit 8,000 characters per second per B channel instead of the 7,000 possible with Bit-Robbing. This arrangement is also called 64 Clear Channel Capability (64CCC).
The DS-0 that carries all of the signaling information is called the D or Delta Channel. This channel will carry only signaling information in a voice or voice and data application. A very limited use of this is all data in which there is no D channel.
Hope this helps.
Matt
